1. Field of the Invention
The present invention relates to an image forming apparatus for visualizing an electrostatic image, formed on an image bearing member for example by an electrostatic recording method, with a developer thereby obtaining a visible image. The image forming apparatus includes, for example, a copying machine, a printer (such as an LED printer or a laser beam printer), a facsimile apparatus, and a work processor.
2. Related Background Art
In an image forming apparatus of an electrophotographic method, an image bearing member such as an electrophotographic photosensitive member is charged and then exposed to light according to image information, whereby an electrostatic image is formed. Such electrostatic image is formed, by the supply of a developer to the image bearing member by developing means, into a developer image. Then such developer image is transferred by transfer means such as a transfer charger onto a recording material such as a recording paper, an OHP sheet or a cloth, and is fixed to the recording material by a fixing device to obtain a recorded image.
For example, an image forming apparatus, utilizing a contact charging apparatus as the charging means for charging the image bearing member, and a non-magnetic one-component contact developing apparatus as the developing means, is already known. A contact developing method employed in such contact developing apparatus is different from so-called jumping developing method in which a gap is formed between the image bearing member and a developer carrying member and an alternating electric field is applied therebetween to execute a development by causing a toner to fly across such gap, and executes a development by contacting the image bearing member with the developer carrying member. This method is becoming practiced recently because of advantages, generating little scattering of the developer at the developing step and capable of forming a developer image faithfully to the electrostatic image on the image bearing member.
FIG. 3 illustrates an example of such image forming apparatus Y. The image forming apparatus Y is constructed as a laser beam printer, and is equipped with a drum-shaped electrophotographic photosensitive member 1 (hereinafter called “photosensitive drum”) as an image bearing member. The surface of the photosensitive drum 1 is uniformly charged by applying a predetermined charging bias to a charging roller 10, serving as a contact charger and contacted with and rotated by the photosensitive drum 1.
Then the surface of the photosensitive drum 1 is exposed, by an exposure apparatus 7 such as a laser beam scanner, to a light L corresponding to an image information signal, whereby an electrostatic latent image is formed. The electrostatic latent image formed on the photosensitive drum 1 is then rendered visible as a toner image, by the supply of a toner as a developer by a developing apparatus A. The toner is for example constituted of spherical toner particles excellent in a transfer property.
The developing apparatus A is equipped, in a developing container B, with a developer roller 2 constituting a roller-shaped developer carrying member, a developing blade 5 constituting a blade-shaped developer regulating member, and a toner supplying roller 3 constituting a developer supplying member. A toner contained in the developing container B is supplied, along with a rotation of the toner supplying roller 3 in a direction as indicated by an arrow, onto the surface of the developing roller 2. The toner on the developing roller 2 is supplied, along with the rotation of the developing roller 2 as indicated by an arrow, to a contact portion between the developing blade 5 and the developing roller 2, then given a triboelectric charge in this portion, and made into a thin layer of an appropriate amount. In this manner the developing blade 5 regulates a thickness of a toner layer carried on the developing roller. The toner made into a thin layer is further supplied, along with the rotation of the developing roller, to a contact portion with the photosensitive drum 1, thereby executing a development according to the electrostatic latent image.
On the other hand, a recording material P is supplied from a recording material cassette 18, through conveying means, to a transfer portion in which a transfer roller 11 is provided.
The toner image on the photosensitive drum 1 is transferred, under an application of a transfer bias (voltage) to the transfer roller 11, onto the recording material P, thereby forming an unfixed image thereon. Then the recording material P is conveyed to a fixing apparatus 6, which fixes the unfixed image to the recording material P, and is further conveyed to a sheet discharge tray 25, whereby the image formation is completed.
Also a transfer residual toner, remaining on the photosensitive drum 1 without being transferred onto the recording material P at the image transfer, is recovered by a cleaning blade 8, constituting cleaning means, into a waste toner container 9. Thus the surface of the photosensitive drum 1 is cleaned and is used again for image formation.
As explained above, the toner in the developing apparatus A is subjected, on the developing roller 2, to a friction with the toner supplying roller 3 and the developing blade 5, and also subjected, at the developing operation, to a friction with the photosensitive drum 1. The toner, after such repeated frictional actions, assumes a state different from a new toner. More specifically, an additive such as silica, externally added to the surface of the toner particle, may be embedded into the toner particle itself or may be liberated therefrom. Therefore the toner shows a gradual change in properties required for the developer, such as fluidity and a charging property. Such phenomenon of changes in the physical properties of the toner becomes eminent when the image forming operation is repeated many times.
According to the studies undertaken by the present inventors, the toner with such change in the physical properties (hereinafter called “deteriorated toner”) causes, when mixed with a toner with a lower level of deterioration in the developing apparatus, induces an agglomeration of both toner particles because of the deterioration in the charging property, fluidity and the like. Also such agglomeration may result in density unevenness in an image and a toner dripping (an agglomerated toner block dripping onto an image in spot-like manner).
For avoiding such density unevenness or toner dripping, there can be conceived a method of reducing a frictional load on the toner, or a method of rendering the toner particle itself resistant to the property changes. However, the former method involves a basic construction such as a triboelectric charging by the developing blade or a contact with the developing drum, so that the extent of reduction in the frictional load has to be limited. Also the latter method is inevitably limited in consideration of assuring necessary properties for the toner, such as fluidity, triboelectric chargeability and fixing property.
In consideration of such situation, other solutions are being desired for such density unevenness and toner dripping, and for example Japanese Patent Application Laid-open No. H09-127771 proposes, in a developing apparatus, to form an alternating electric field between the developing roller and the developing blade. This developing apparatus causes a reciprocating motion of the toner between the developing roller and the developing blade to disintegrate the agglomerated toner thereby preventing the density unevenness and the toner dripping and stabilizing the toner coating on the developing roller.
Also U.S. Pat. No. 5,519,472 proposes, in a developing apparatus, to form an alternating electric field between the developing roller and the developing blade, thereby preventing an uneven charging of the toner and an uneven coating of the toner on the developing roller.
In the contact developing method, however, an alternating electric field formed within the range as described in Japanese Patent Application Laid-open No. H09-127771 between the developing roller and the developing blade may be unable to avoid the toner dripping or the density unevenness with a toner in a state with changed physical properties after repeated image forming operations. Also with the alternating electric field formed within the range as described in U.S. Pat. No. 5,519,472 between the developing roller and the developing blade, the toner coating state on the developing roller may still show a fluctuation, thus resulting in a decreased toner amount on the developing roller and giving a low density. Also when the image forming operation is repeated many times, the toner may sticks to the developing blade, thereby resulting in a longitudinally streaked image, called a development streak.
Also in the contact development method, when the developing apparatus is left standing for a prolonged period, the developing roller may become locally recessed, in a contact position between the developing roller and the developing blade, by a pressure of the developing blade, whereby a toner coating amount on the developing roller may become increased in such recessed portion, thus giving rise to a banding image in which the image density becomes higher at a cycle of the peripheral length of the developing roller (such image being hereinafter called “developing roller set mark image”).